Luminescent tube system



March 1945. J. H. BRIDGES 2,370,635

LUMINESCENT TUBE SYSTEM Filed June 25, 1942 IN VEN TOR JoH/v file/e011; flzloses fireman 6,1945

LUMINESCENT TUBE SYSTEM John Herold Bridges, Paterson, N. 1., alsignor to Boucher Inventions, Ltd., a corporation of Delaware Application June25, 1942, Serial No. 448,471

13 Claims. (Cl. 315247)' My invention relates to fluorescent lighting equipment, and more particularly concerns a new transformer power unit and associated system,

' for powering fluorescent space discharge tubes.

An object of my invention, therefore, is to produce a simple, compact, sturdy, reliable and inexpensive transformer, having high leakage reactance, characterized by the small iron content required, its low operation and maintenance, costs, and its good voltage regulation.

Another object is to produce an autotransformer embodyingall of the advantages set forth in the foregoing, and the windings of which are disposed in the smallest possible compass.

A still further object is to produce a new fluorescent tube'lighting system embodying a transformer power unit having the advantages described, and possessing the qualities of quick striking, long tube life, steady tube operation in the substantial absence of detrimental flicker even during cold weather operation, the substantial elimination of undesired stroboscopic effect, and

high system power factor.

Other objects and advantages will in part be Figure 2 depicts the electrical circuits according to my invention; while Figures 3 and 4 are graphs illustrating wave forms on the condensive and inductive sides, respectively, of the system according to my invention.

To facilitate more thorough understanding of my new transformer and system. it may be noted that the illumination art has come more and more to accept fluorescent tube lighting as a customary and desirable expedient in a number of widely diversified fields involving both general and specialized illuminating problems. Many factors contribute to this rapid acceptance of fluorescent lighting. Among them may be cited, simply by way of illustration and by no means intended as exclusive, the high lighting efficiency of such tubes for unit power input. whereby maximum light output can be achieved at a minimum of cost; as well as the low temperatures at which such tubes operate, whereby in entirely practical manner, high intensity lighting of comparatively low brilliance can be achieved in enclosed spaces.

Because of the economies of such fluorescent lighting 'from a power standpoint, it makes entirely feasible, for the first time, the use of adequate lighting for advertising and other purposes, in those cases where for economic reasons, adequate illumination was hitherto impossible. The economy of such lighting is contributed to by the long life of the fluorescent tube, which at the present time, is rated at between 2 to 2 /2 times longer life than are the best grade of incandescent lamps now on the market.

The active light emission region of these tubes being long as compared to the light source in incandescent bulbs produces an intense but diffused light of low brilliance which lends itself admirably to flood light eifects. It is entirely possible, because of the low operation costs and the low temperatures at which the tubes operate, to employ them in batteries of two, three, four, or more tubes. Thus arranged, they may be employed in extremely satisfactory flood lighting displays. Their use has become widespread in both indoor and outdoor fields where daylight conditions must be approximated, as for example, in factories, filling stations and the like.

It is possible to vary the characteristic color of particular tubes within wide limits to satisfy the particular lighting problem in question, simply by employing diflerent fluorescent salts or phosphors as liners for the tubes, the particular visible secondary, radiation of such salts having pre viously been determined and calibrated by experiment. It is this feature, widely at variance with the substantially fixed quality of radiation avail able from the known filamentary lamp, which has constituted an important factor contributing to the present-day widespread use of fluorescent lighting in advertising, store and household light-= mg.

Despite, however, the rapid growth of fluorescent tube lighting practice, to its present day important position in the illumination field, many defects, serious from a practical standpoint, re-

main to be removed before fullest exploitation of Difliculty is encountered, however, in that these low voltage tubes do not start with any degreeof facility on the ordinary service potentials. Consequently, it frequently becomes necessary to equip them with an additional starting electrode, and in all instances a starting switch must be supplied for each such low potential tube. Such switches ordinarily take one of three conventional forms: the magnetic relay, the thermal, or the glowdischarge type. All of these switches are found to be comparatively slow in starting, requiring about six seconds on the average to strike the arc, while in the glow-discharge type of switch, diniculty is experienced in quickly re-striking the arc, once it has extinguished for any reason, after being initially struck.

Additionally, where several tubes are operated in parallel, a starting compensator is found to be required, disposed in series circuit with that tube of leading current demand, to ensure that suillcient voltage is applied initially to strike the arc across that tube. Finally, these are discharge tubes have negative resistance characteristics, by which is meant that once the arc is struck, the resistance of the discharge path thereacross will decrease to such extremely small value that unless adequate current-limiting means are provided, the current across the arc will increase to a prohibitive value, either destroying the tube load or burning out or interrupting the electrical energy supply thereto. In low-voltage tube systems essential current-limiting means usually takes the form of an ohmic resistance on the less expensive installations, or of an iron-core choke coil in the more costly units.

All of these auxiliaries, as they are called, require complicated and expensive installation technique and demand many electrical connections. Their large number of auxiliaries for a battery or bank of tubes results in a bulky assembly, neat appearance of the complete assembly and. simplicity in installation being difflcult, indeed, well-nigh impossible to achieve. Each such auxiliary separately constitutes a po tential source of disturbance in the system and is subject to getting out of order or failure, giving rise to attendant maintenance or repair costs. Too, it is evident that although the cost of each auxiliary may in itself be a comparatively small item, nevertheless when it is considered that it is customary to employ such tubes in banks or batteries of two or more, and that despite this, each tube still requires its own complement of auxiliaries, it is at once evident that the total cost cannot escape being high.

Furthermore the comparatively small iron content of the iron core ballasts and the absence of iron in the ohmic resistors, result in poor voltage regulation, so that the tube equipment is extremely sensitive to variation in line voltage.

The low terminal voltage at which these hot cathode tubes are operated gives rise to the serie disadvantages in cold weather operation of either detrimental flicker, or in extreme cases, of extinguishment f the arc. For like reason, it is diflicult or even impossible, where low-potential, hot-cathode tube equipment is employed, to strike the are initially when extremely cold weather conditions prevail. This undesirable phenomenon is attributable to the fact that the striking potential of the tubes is close to the peak voltage obtainable from the ordinary service mains, whereby no reserve of impressed voltage is present to accommodate even slight increase over rated value of the striking potential of the arc.

Experience demonstrates that hot-cathode tubes employed in hot-cathode operation in lowvoltage systems are comparatively short-lived,

asrqess this short life being contributed to by a number of causes. Among them may belisted by way of illustration, the inherent fragility of the incandesciblefilaments employed. It is wellrecognized to be characteristic of such filaments that in operation, the arc tends to settle locally at one point thereon, usually the high voltage end. Burning follows, shortly attended by failure of the electrode, whereupon the tube displays rectifier action, attended by continuous flicker and usually followed by early failure of the other electrode.

Additionally, reliance is placed in striking the arc across these low-voltage, hot-cathode tubes, on copious emission of electrons from the electrodes thereof. To this end, the electrodes are customarily coated with a suitable electron-emitting substance, usually a suitable oxide. During operation, however, this electron coating is gradually dissipated, so that for this, a well as for the other reasons set forth hereinbefore, the lowvoltage, hot-cathode tubes do not display the long life which is desirable in this art.

Considerable research has been carried out by various workers skilled in the art, looking for one or more solutions for the various problems set forth in the foregoing. Among the more important developments, that of materially increasing the terminal voltages at which the fluorescent tubes are energized, at the same time employing them in cold-cathode operation, is outstanding. It has been proposed that those elevated voltages be transformed directly through the intermediary of a transformer from service mains of conventional voltage rating, such as or 220 volts.

For such transformer system operation, either a specially designed cold-cathode tube or an adapted hot-cathode tube is employed. Hotcathode tubes, which are readily available on the market, are conveniently adapted for coldcathode operation simply by short-circuiting the terminals of the electrodes thereof to ensure that all portions of a particular electrode are maintained at the some potential. It was found that by observing proper precautions in the operation of these hot-cathode tubes on cold-cathode operation, they display great ruggedness, and their effective lives are prolonged considerably over what had hitherto been experienced.

To economize on both space and materials required, and in general to reduce production cost to a minimum, recourse preferably has been had to autotransformer equipment. Up to the limiting voltage permitted by the Fire Underwriters, such transformer equipment is entirely satisfactory, although slightly higher in first cost than is the conventional hot-cathode tube lighting unit, including its necessary auxiliaries. The sturdiness of such equipment and the small amount of attention and supervision required thereby make its total cost, including maintenance, only a little, if any, greater than the comparable cost of the hot-cathode equipment. When such transformers are employed, either as single or double transformers, i.e., transformers having one or two separate secondaries, to energize banks of two, three or more tubes, then the absence of the hitherto all-essential auxiliaries is found to bring the unit cost to a value lower than that of the hot-cathode tubes.

It is not sufficient, however, simply to interpose a transformer between the line and the fluorescent tube load, because such ordinary transformer, either with simple or autotransformer connection, does not provide sufficient impedance to revent enormous build-up of current and voltage across the secondary, once the arc across the tube is struck. On the other hand, if recourse be had to auxiliaries to ballast the tubes, then one of the main purposes of employing transformer energization means, namely, keeping the over-all cost to a minimum, would be defeated. This difficulty has been alleviated by providing in the iron core of the transformer, between the primary and secondary windings, high leakage reactance shunts. These shunts normally have no effect when the secondary windings are de-energized, so that substantially all of the primary flux from the primary winding courses along the main magnetic path or paths, interlinking the secondary winding or windings. However, when the arc strikes across the load consisting of one or more tubes, changing the reluctance of the main flux path, these shunts then display a reluctance lower than that of the main magnetic path. This change in reluctance of the main magnetic path results from the back magneto-motive force set up by the energized secondary winding. The main stream of primary flux then courses across these high reluctance magnetic shunts, with their included air-gaps, the calibration of the shunts being such that just suflicient primary flux interlinks the secondary winding or windings to induce a voltage and current therein sufficient to maintain the arc across the tube load.

Such new tube lighting equipment, consisting of a transformer fluorescent tube lighting sysfem and associated power unit, displays many extremely important and thoroughly practical advantages as contrasted with the lower voltage. hot-cathode tube lighting units theretofore employed. The arc across the tube displays quick striking characteristics, and can readily be restruck, should it momentarily extinguish for any reason. The number of parts is reduced to a minimum. A far smaller number of external connections is required than had hitherto been the case. The power unit is simple and of small size. The sturdiness of the lighting unit, even with outdoor use and while subjected to severe weather conditions. is found to be outstanding.

The higher rated voltages make it entirely feasible. from a practical standpoint, to operate the tubes under dimmer conditions, i. e., at primary terminal voltages which are lower than the rated terminal voltages. The higher secondary voltages, substantially in excess of the striking potential of the tube, permit a reduction in the secondary voltage, while still maintaining the peaks of the second voltage wave form substantially in excess of the striking potentials of the tubes. When so operated, the period during which the arc remains energized across the tube during each current half-cycle is decreased, so that the total quantity of light per unit of time is diminished, thus giving rise to the dimmer action referred to.

Many further refinements and improvements upon the transformer-powered cold cathode fluorescent tube lighting systems of the type described, however, remain to be accomplished. before the full field of utilizationof this equipment can be exploited. For example, while the life of the tubes employed in transformer-powered units of the type described is satisfactory, measured on an absolute basis, the results nevertheless are somewhat disappointing in that this life is not quite as great as appears to be indicated, from theoretical considerations. The decrease in tube life, determined experimentally, from the anticipated ideal value constituted a most perplexing problem, no satisfactory solution for which was brought forward for a considerable time span. Finally, however, it was discovered that the double transformer,

which had been used in so many lighting systems to achieve economies therein, was probably at fault. The'combination of capacitative, and inductive reactances, constituted by the tube load and secondary winding, frequently produces resonance effects in one or both secondaries which are then transmitted through the autoconnected primary winding to the other secondary winding, creating disturbances and surges therein. The effect is cumulative and reciprocal, so that after the passage of a number of current cycles, extremely high resonating and transient voltages are built up. which particularly display themselves at the rztl'oments of striking and extinguishment of the arc during each current half-cycle. These high voltages have very disturbing eifects on the tube, decreasing the life thereof appreciably, sputtering the electrode materials and destroying the electrodes themselves, depositing the sputtered electrode material on the walls of the tube so as to coat the fluorescent lining thereof serving as a trap in which to occlude the gas content of the tube, resulting in "hardening of the tube, decreasing the electrode emissive qualities of the sputtered electrode, and perhaps eventually destroying either the tube or the associated equipment by puncturing the insulation thereof.

An important object of my invention, therefore, is to eliminate in large measure the undesirable transient waves in transformer equipment referred to, and to produce a new transformer power unit and it new fluorescent tube lighting system which are characterized by their uniformly good wave form, the substantial absence of detrimental high-voltage transients, and by the long life of the tube units employed in such system.

Additionally, appreciable reduction in the iron content required of the transformer unit would result in appreciable savings in the cost of production, as well as in the size of the unit. This reasonably can be expected to enhance appreciably the acceptance of this type of tube lighting equipment beyond even the already assured and established position} and acceptance which such equipment occupies in the art.

A further object of my invention, therefore, is to produce a transformer having high leakage reactance characteristics and which is adapted for powering fluorescent tube lighting systems, which transformer is characterized by its extremely small size for rated power output and which requires a minimum of iron content for rated efiiciency and rated voltage regulation of the associated load. and which transformer is simple, compact, sturdy and inexpensive, both to produce and to operate, and which at the same time is highly practical and complies fully with all of the requirements of the Fire Underwriters.

Referring now more particularly to the embodiment of my invention depicted in Figure 1, therein the transformer core is indicated generally at H). This core consists illustratively of a central, longitudinally extending leg H, flanked on oppo site sides, preferably at equal distances therefrom, by two similar outer legs l2 and I3 extending parallel thereto. Corresponding ends ofthe outer legs and central leg are closed by similar outer legs or end pieces I4 and I5.

While as has been described, and for purposes of symmetry, and so that the flux courses through the branch magnetic paths to be described will be substantially equal, I prefer to space the outer legs of similar magnetic proportions equally distant from the central leg II, it is not absolutely essential that this be done, and it is entirely possible that the spacing or magnetic properties of any or all of the outer legs, as well as the inner leg II, can be increased or decreased within wide limits. In point of fact, an operable structure can be achieved with the removal of one of the outer legs in its entirety. However, I prefer to construct the core so as to produce the balanced, shell-type transformer illustrated, giving a transformer which is reduced to the smallest possible compass consistent with good magnetic and electrical characteristics and performance. Intermediate the lengths of the outer legs I 2, I3, I provide high leakage reactance magnetic shunts S711 and Shz, respectively, extending towards but short of the central leg II, forming therebetween air-gaps G1 and G2. Preferably, these shunts, as well as the other parts of the core structure, are constructed of laminated iron material.

The air-gaps G1, G2 are constructed to have a desired and predetermined high reluctance, calibrated in accordance with the admittance of that part of the main flux path illustrated at the left of the shunts in Figure 1, when that main flux path is operating under load conditions. More will be said about this action at a later point in this description.

The shunts have the effect of dividing the openings provided between central and outer legs into two pairs of spaces, one pair on each side of said shunts. In one said space, to the right of the shunts in Figure 1, I provide a primary winding I5, about the central leg II, which said winding is energized through leads I! and I8, by a suitable source of alternating-current energy I9.

Since it is desirable to energize the system from the ordinary service mains, which usually are of approximately either 110 or 220 volt rating, and since I desire to operate my tubes at the maximum of 600 volts secondary terminal voltage permitted by the Fire Underwriters for autotransformer-connection, or even at higher voltages when the autotransformer-connection and its resulting copper savings are dispensed with, and general transformer connection resorted to, I construct my transformer of the step-up type, with the effective secondary windings having a substantially greater voltage per turn than does the primary winding.

I provide my transformer with two secondary windings, one in each of the mentioned pair of spaces. The first secondary winding, indicated at 20, preferably is wound, as shown, directly on the primary winding I6, thus effecting material economies in core material and in the total space requirement of the power unit. It is entirely feasible, if desired, however, to elongate legs II, I2 and I3, and to position the secondary winding 20 at the side of the primary winding, separate therefrom, but within the first pair of spaces as thus elongated. It is evident that this primary winding in the embodiment shown, is electrically separate and independent of the secondary winding 20, and has no direct electrical connection therewith. This secondary winding 20 is only inductively associated with the primary winding I6. Additionally, with the secondary winding 20 being mounted on central leg II and on the same side of shunts Shi, Sh: as is the primary winding Hi, the shunts exert no current-limiting effect when the arc strikes across the tube load of this secondary winding. A power-factor correcting condenser C advisedly is employed where it is desired to restore the system power-factor to approximately unity. Reliance also is placed upon this condenser for ourrent-limiting action in the load circuit of secondary coil section 20, as is more fully described hereinafter.

In the second pair of spaces, a second secondary winding 2| is provided, on the opposite side of shunts Shr, Sh: from the primary winding I8, and this secondary winding 2| is connected in autotransformer connection across the primary winding I6. Circuits may be traced from primary winding I6 to secondary winding 2| as follows: from the left hand end of winding IS in Figure 1, down through lead I! to junction 22; thence through lead 23 to the right-hand end of secondary winding 2| in Figure 1, to the left across this secondary winding to the left-hand terminal thereof, then down through lead 24 t0 tube T2, lead 25, junction 26, and up through lead I8 to the right-hand end of primary winding I5.

It may be pointed out at this time that secondary winding 20 directly energizes tube T1 and condenser C through leads 2'! and 2B. A circuit may be traced from the left-hand end of winding 20 in Figure 1, lead 28, tube T1, condenser C and lead 21 back to the right-hand end of secondary winding 20.

The source IQ of alternating current electrical energy energizes winding I6 through a circuit of periodically reversing direction, traced as follows for a given current half-cycle: to the right in Figure 1 from source I9, up through lead I8, to the right-hand end .of winding I6, across the winding and back through lead I! to the lefthand end of the power source. During the next successive current half-cycle, of course, the direction of current flow is just the reverse of that traced.

It is interesting, as well as highly instructive, to consider at this point, the manner in which the primary flux courses the flux paths through the illustrative magnetic core which has been described. For this purpose, consideration is given to an instantaneous direction of flow of primary current through winding I6 such that the primary flux developed thereby courses to the right in Figure 1 along central leg I I from winding I6. When the flux reaches end pieceid, it courses along two individual paths of low reluctance, one said path sequentially including end piece I4, core leg I2, and end piece I5 to leg II. Similarly, the second flux path sequentially includes end piece I4, core leg I3, end piece I5 to leg II.

' Since the flux seeks paths of least reluctance, the high reluctance magnetic by-passes Shi, G1 and Shz, G2, respectively are in large measure avoided, and only an extremely small part of the flux courses across these shunt paths, to the central leg II, and back to primary winding I6. As has been stated, by far the greater part of the flux courses along the two individual paths of low reluctance; which paths reunite at the lefthand end of inner core leg II, and return along leg II to primary winding I6. It will be noted that at all times the full quantity of primary flux links the secondary winding 2|, and develops a rated high voltage therein. Similarly, before the secondary tube load T2 across winding 2| has been energized as a result or the arc striking thereacross, either initially or during any current halt-cycle, substantially the full primary flux links winding 2|, inducing a high voltage therein.

Because of the high secondary voltages employed, as has been stated hereinbeiore, the terminal voltages oi the secondary windings, impressed directly across the electrodes of the associated tube loads, bring these tubes to a high de-. gree oi! excitation so that after the passage of but a comparatively few cycles of current flow, the arcs in the several tubes ignite, and steady operating conditions maintain.

As soon as the tube load T1 becomes energized, a back magneto-motive force is developed in winding 20, tending to buck the primary flux from 16. Because no magnetic by-pass is provided be tween windings 20 and I6, flux continues to course the main magnetic core.

When the arc strikes across tube T2, the back magneto-motive force developed in winding 2| is found to afford considerable reluctance to the coursing of the primary flux along the path encircled by winding 2|. Always seeking the path or paths of least reluctance, therefore, and since the magnetic by-passes Shi, G1, Sha, G2 are now or lower reluctance than the main path interlinking winding 2I, the flux coursing the individual magnetic paths, travels down shunt leg 571.1 and across air-gap G1, and up shunt leg She and across air-gap G2, respectively, directly back to primary winding I6, and in large measure bypasses winding 2|. At such times when the arc is struck across tube Ta and the tube remains energized, the shunts because of their design, permit only enough primary flux to interlink winding 2| to provide suflicient voltage andcurrent to ensure energization of tube T2.

During the next subsequent and alternate halicycle of primary current how, the direction of coursing of the primary flux is just the reverse of that which has been traced. From winding I8, the flux courses along leg II to the left in Figure 1. Very small quantities of flux course, across air-gap G1, up shunt Sh1, to the right along leg 52 and down end piece I4, leg I I, back to winding i5; and down across air-gap G2, shunt leg Shz. to the right along leg I3, up end piece I4, leg I I, and back to winding I6. By far the greater part continues to the left along leg II, interlinking winding 2i and inducing a high voltage therein. At end piece E5, the main flux separates into two substantially equal branches. The flux courses, in part, up end piece ii, to the right in Figure 1 along leg I2, down end piece I4, to leg II; and, in part, courses down end piece I5, to the right in Figure 1 along leg I3, and up end piece I4 to leg II. There the flux reunites and courses along leg I! back to winding I6, interlinking winding 20. when the are initially strikes across tube T1, and when it strikes anew in each current halfcycle, the back magneto-motive force developed thereby has relatively little effect on the primary flux, for the reasons pointed out hereinbefore.

However, soon as the arc strikes across tube T2, a back magneto-motive force is developed in coil 2| in a direction contrary to and bucking the primary flux, so that the main flux path, previously oi reluctance, now becomes one of comparatively high reluctance as compared to the high leakage shunt paths including shunts Shi and Ski. Since the flux always seeks a path or paths of least reluctance, by far the larger part of the primary flux at this time courses across two shunt paths, effectively by-passing in large measure the secondary winding 2|. Flux courses, in part, up across air-gap G1, Shi, to the right along leg I2, down end piece I4, and back through leg II to winding I6; and courses, in part, down across air-gap G2, Shz, to the right along leg I3, v

up end piece I 4, and back along leg II to winding I5. Only a small amount oi the primary flux, sufiicient to maintain the voltage and current required for tube T2, continues coursing the length of leg II, interlinking winding 2| and splitting at end piece I5 into two branches which course back to winding I6 through legs I2 and I3 in the manner previously discussed.

Toward the end of each half-cycle of primary charging flux, as the value of the flux decreases, the induced voltage drops to a point where the arc across the associated tube load extinguishes, whereupon the back magneto-motive force at once disappears. The reluctance of the main flux paths drops to its former value, so that the flux no longer courses the high leakage reactance shunt paths, but resumes coursing the main flux paths until at such times as the arcs are reignited.

Tubes T1 and T2 are of the usual fluorescent gas discharge type in that they are designed so that a, large part of the input energy is converted to what is known as the resonance line, 2537 A., in the ultra-violet portion of the tube spectrum. This radiation serves to excite the particular photoelectric coating or lining, known as a phosphor, provided on the interior walls of the tube, and which energized, gives rise to a desired characteristic radiation. As stated, the tubes may either be the available hot-cathode tubes now on the market, with short-circuited cathodes to ensure uni-potential electrodes, or cold-cathode tubes designed especially for the particular use to which they are to be put.

Since secondary winding 20 from a magnetic standpoint is disposed on the same side of shunts Shi, G1, and Ski, G: as is winding I6, and is in only inductive association with the primary winding, the transformer, certainly within the saturation limits of the iron core thereof, exercises no appreciable control action on the build-up of current flow across the tube load of negative resistance characteristics. However, since the predominately inductive nature of the load across source I9 requires the use of a balancing capacitative reactance to restore somewhat the system power-factor, this capacitative reactance, in the form of high capacitative condenser C, is disposed across secondary 20, in series with tube T1; and use is made of its high reactance to maintain the current across this tube within the required safe limits. It is for this reason that it is unnecessary to dispose a high leakage reactance shunt or shunts between windings I6 and 20. This condenser, for example, may have a capacity ranging from 1.5 to about 1.75 micro-farads.

In operation, probably because the winding 20 is not in autotransformer connection with primary I5 but is only inductively associated therewith, disturbances arising in one of the secondary circuits are substantially damped from the other secondary circuit. Thus, in a lighting system according to our invention, all substantial tendency towards cumulative resonating efiects or transient disturbances between transformer secondary windings is substantially suppressed, giving improved light emission and prolonge tube life.

In Figure 3, for example, a graph is shown of one cycle of the current flow on the T1 or condenser side of the transformer system, while in Figure 4 is displayed one cycle on the T2 or inductive side of the line. In the latter case, the wave form is nearly sine-wave in nature. In each case, the abscissae are current values, while the ordinates are units of time. Substantially all ripples at the striking and extinguishment of the are are avoided by directly shunting tubes T1 and T: by small, radio frequency condensers 23, 30, respectively, of approximately. .005 or .006 micro-farads capacity.

I have found that my new construction makes it possible to retain substantially all of the advantages of the high leakage reactance transformer powerunits for fluorescent tube lighting, such as quick starting and stable operating characteristics, even under extreme weather conditions which may prevail, long tube life, good voltage regulation, economy both in first cost and in operation and maintenance expenses, simplicity, sturdiness and ruggedness, while at the same time achieving a thoroughly practical power unit and associated system in which the iron content is reduced to a minimum with no decrease in good voltage regulation. The resulting unit is small, compact, requires a minimum of space and conveniently is mounted, for example, directly on the reflector of a tube assembly, and has a minimum number of external connections.

The new system which I produce has high system power factor, so that energy consumption, and hence costs, is reduced to a minimum, and waste power, dissipated in heat, is negligible. Transient harmonics of any appreciable or detrimental values are substantially eliminated, so that a smooth, almost ideal wave form results, with no detrimental over-voltages, and so that both the new transformer and system comply fully with the requirements of the Fire Underwriters.

Attributable in large measure to the substantial absence of high voltage harmonics and the consequent presence only of rated voltages, the tube life is greatly prolonged, and the electrode material thereof is unharmed by sputtering or similar phenomena. Little if any electrode material is volatilized oil and deposited as an opaque lining on the tube walls, so that light transmission remains substantially constant throughout the normal life of the tube. Similarly, there is no occlusion of the gas content of the tube by any such deposited coating, as a result of which there is-a substantial absence of the detrimental phenomenon known as hardening. No appreciable decrease in the electron-emitting qualities of the electrodes is experienced during the rated life of the tubes. These several last-mentioned advantageous features contribute to light emission of substantially uniform quality throughout the useful life of the tubes.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative, and not in a limiting sense.

I claim:

1. An electric lighting system, comprising, in combination: a transformer having a primary winding and two secondary coil sections, one of said secondary coil sections being electrically independent of the primary winding and the other being electrically connected in autotransformer relationship therewith; a luminescent lamp load connected across said electrically interconnected primary winding and secondary coil section; another luminescent lamp load and a current-limiting condenser connected in circuit with said electrically independent secondary coil section; and by-pass means of high reluctance magnetically separating said primary winding and electrically independent secondary coil section from said other secondary coil section.

2. An electric lighting system, comprising, in combination: a transformer having a primary winding and two secondary coil sections, one of said secondary coil sections being electrically independent of the primary winding and the other being connected in autotransformer relationship therewith; a luminescent lamp load connected across said electrically interconnected primary winding and secondary coil section; another luminescent lamp load and a current-limiting condenser in circuit with said electrically independent secondary coil section; radio frequency condensers directly shunting said luminescent lamp loads; and by-pass means of high reluctance magnetically separating said primary winding and electrically independent secondary coil section from said other secondary coil section.

3. An electric lighting system, comprising, in combination: a transformer having a primary winding and two secondary coil sections mounted on an inner core portion, one of said secondary coil sections being electrically independent of the primary winding and the other being connected in autotransformer relationship therewith; a luminescent lamp load connected across said electrically interconnected primary winding and secondary coil section; another luminescent lamp load and current-limiting condenser connected in circuit with said electrically independent secondary coil section; an external transformer core portion forming magnetic paths with said inner core portion on opposite sides thereof; and shunt means forming with said inner and outer core portions a by-pass of high reluctance magnetically separating said primary winding and electrically independent coil section from said other secondary coil section.

4. An electric lighting system, comprising, in combination: a transformer having a primary winding and a secondary coil section electrically independent thereof disposed in superimposed relationship on a main magnetic core, and another secondary coil section mounted on said core and connected in auto-transformer relationship with the primary winding; a luminescent lamp load connected across said electrically interconnected primary winding and secondary coil section; another luminescent lamp load and a current-limiting condenser connected in circuit with said electrically independent secondary coil section; and by-pass means of high reluctance magnetically separating said primary winding and electrically independent secondary coil section from said other secondary coil section.

5. An electric lighting system, comprising, in combination: a transformer having a primary winding and a secondary coil section electrically independent thereof disposed in super-imposed relationship on an inner magnetic core portion, and another secondary coil section mounted on said inner core portion and connected in autotransformer relationship with the primary winding; a luminescent lamp load connected across said electrically interconnected primary winding and secondary coil section; another luminescent lamp load anda current-limiting condenser connected in circuit with said electrically independent secondary coil, section; an external transformer core portion forming magnetic paths with said inner core portion on opposite sides thereof; and shunt means forming with said inner and outer core portions a by-pass'of high reluctance magnetically separating said primary winding and electrically independent coil section from said other secondary coil section.

6. Apparatus for operating two fluorescent tube loads, comprising in combination, a primary winding and a secondary coil section disposed in superimposed relationship, and another secondary coil section spaced therefrom; leads from said coil sections for individual connection to said tube loads; a main magnetic core linking said primary winding and the secondary coil sections; shunt core means forming with said main magnetic core a by-pass of high reluctance magnetically between the primary winding and said spaced secondary coil section for controlling the current in said'coil section; and a condenser connected in series with said superimposed secondary coil section for controlling the current therein to like extent.

'7. Apparatus for operating two discharge tube loads, comprising in combination, a magnetic core of the shell type having an inner leg and two outer legs; a primary winding and a secondary coil section disposed closely adjacent thereto, and

another secondary coil section spaced therefrom,

all mounted on the inner leg of said core; leads from said coil sections for individual connection to said tube loads; core shunt means with in cluded air-gaps forming-with said inner and outer core legs a by-pass of high reluctance magnetisaid tube loads; a current limiting condenser connected in series with one of said secondary winding coil sections to control the current therein; and by-pass means of higher reluctance magnetically separating the other secondary winding from said primary winding and said latter secondary winding coil section to control the current to like extent in said other winding.

9. An electric lighting system, comprising. in combination, a transformer having a primary winding and two secondary winding coil sections. with at least one of said coil sections connected in autotrsnsiormer relation with said primary windinl, two luminescent tube loads individually ccnnectedincircuitwithsaidtwosecondsrycoii sections. condenser means connected between one tube load and its associated secondary windin coil section for controlling the current therein, and said transformer including a magnetic core shunt between primary winding and said other secondary winding coil section for controlling the current therein to like extent.

10. Apparatus for operating two discharge tube loads, comprising, in combination, a transformer having a primary winding and two secondary winding coil sections, with at least one of said coil sections being connected in autotransformer relationship with said primary winding and with leads for individual connection to said tube loads, a current limiting condenser connected between one secondary winding coil section and its associated lead for limiting the current in said coil section, and magnetic core shunt means between the primary winding and the other secondary winding coil section for limiting the current therein to substantially like extent.

11. A fluorescent tube lighting system, comprising a high leakage reactance transformer having a primary winding and two econdary windings mounted on a common core, a fluorescent gas discharge tube connected in circuit with one of said secondary windings, a second fluorescent gas discharge tube connected in circuit with the other of said secondary windings, shunt core means magnetically between the primary winding and one only of said secondary winding to limit the current in said winding, and a condenser in circuit with said other of said secondary windings and the tube connected thereto to limit the current in that winding to substantially that of the first-mentioned secondary winding.

12. Apparatus for operating two discharge tube loads, comprising in combination, a transformer having a primary winding and two secondary winding coil sections, lead from said secondary winding coil sections for individual connection to said tube loads, magnetic core shunt means separating one secondary winding coil section from said primary winding to limit the current in said coil section and associated lead, and a condenser of such capacity connected between said other secondary windingcoil section and its associated lead as to limit the current therein to substantially the same value.

13. A fluorescent tube lighting system comprising in combination, two fluorescent gas discharge tube loads; a transformer for energizing said loads having a magnetic core of the shell'type with an inner leg and two outer legs, two magnetic core shunts with included air-gaps respectively extending between the inner leg and said outer legs, and a primary winding and two secondary winding coil sections mounted on said inner leg, one of said coil sections being separated from said primary winding by said magnetic core shunts and connected to one of said tube loads, and the other coil section being closely adjacent the primary winding and connected to the other discharge tube load, said core shunts eil'ectively controlling the current in said separated coil section; and a condenser in the circuit with said other coil section and the tube load connected thereto to limit the current in that coil section to substantially that of the shunt-separated coil section.

JOHN HEROLD BRIDGES. 

